The present invention relates to a loop type data transmission system in which a plurality of data transmission control stations (hereinafter termed simply as stations) having computers and/or various terminal devices connected thereto are interconnected in cascade through a single loop transmission line, wherein data transmission can be effected between any given ones of the stations.
In the loop type data transmission system mentioned above, only one of the stations is permitted to transmit data at one time. Consequently, control must be made in such a manner that the station demanding data transmission may first acquire the permission to reserve the line and upon completion of the data transmission, the line is released and transferred for reservation by another station.
All the controls involved in acquiring and releasing reservation of the line are herein generally referred to as the line control.
A data transmission control system of control station fixing type in which only a particular station (control station) is imparted with the line control function is certainly advantageous in that all the other stations need not be additionally provided with the respective line control functions. However, this type of data transmission control system can not enjoy a high data transmission efficiency, because every station must inform the control station of completion of the transmission every time the data transmission has been completed. There is suggested in T. Nakano's Japanese Patent Application Laid-Open No. 115105/1977 (Application No. 31312/1976) a data transmission control system of control station turning type in which the line control function is not fixedly imparted to any particular station, but any station having completed the respective data transmission serves as the control station by turns.
Further, a transmission system referred to as the token pass system is disclosed in European Patent Application EP-A No. 0054077 (No. 80107706.6) filed Dec. 8, 1980 and published June 23, 1982.
In the token pass system, token information indicative of the right of transmission is circulated along a loop transmission path. When a transmission station having a request for transmission detects the token, this station fetches the token and transmits data. Subsequently, upon completion of the transmission, the station sends out the token for transferring the right of transmission to another station.
As compared with the control station fixing type transmission system, the control station transferring type system mentioned above is more effective in respect to the transfer of the right of transmission to enhance the transmission efficiency, to an advantage. Further, because there exists no fixed relationship of master and slave among the stations, high degree of dispersion of the transmission control can be realized to another advantage.
However, there arise problems mentioned below in the control station transferring type transmission system when the right of transmission is transferred from the station having completed the transmission to another transmission station.
One of the problems is seen in the fact that, when no transmission stations issue the transmission request, the station having completed the transmission must send out again the returned token (re-sending of the token). Second, in consideration of possibility of disappearance of the token due to transmission failure or disturbance such as noise, the time required for the token to perform a round trip has to be supervised for the re-sending of the token.
Although incorporation of these supervising and control functions in each of the transmission stations is preferable in respect to the dispersion of the transmission control, it is not advantageous in practical applications because of an increase of the hardware required for realizing the transmission stations and complication of the control.